Conventional tripod-shaft portions are formed as a columnar shape. In this a tripod-shaped constant-velocity universal joint, when transmitting rotary force in the case where the joint angle is not 0 degree, the rollers move reciprocally along the roller grooves of the outer race while the rollers rotate around the axis of the tripod-shaft portions and move reciprocally in the axial direction. On this occasion, since the tripod-shaft portions have a columnar shape, the rollers are always positioned on an identical axis with respect to the tripod-shaft portions. Accordingly, slippage occurs between the rollers and the roller grooves; as a result, induced thrust force occurs in the joint-axis direction. The resulting induced thrust force becomes the occurrence cause of vehicle-body vibrations and noises, and have an influence on the noise, vibration, and harshness (NVH) performance of vehicle.
Hence, in order to reduce the induced thrust force, a sliding-type tripod-shaped constant-velocity universal joint is available, sliding-type tripod-shaped constant-velocity universal joint which is disclosed in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 3-172,619, for instance. In this sliding-type tripod-shaped constant-velocity universal joint, the tripod-shaft portions' outer-peripheral-surface configuration is formed as a spherical-surface shape. By means of this, the rollers become rotatable with respect to the tripod-shaft portions, and thereby it is adapted so that no slippage occurs between the rollers and the roller grooves.